1. Field of the Invention
The present invention relates to a method for making a field emission cathode device, and more particularly to a method for making a carbon nanotube-based field emission cathode device.
2. Prior Art
Carbon nanotubes are a novel carbonaceous material discovered by Iijima, a researcher of NEC corporation, in 1991. Relevant information was reported in an article by Iijima, entitled “Helical Microtubules of Graphitic Carbon” (Nature, Vol. 354, P56, 1991). Carbon nanotubes can transmit an extreme high electrical current and emit electrons at a very low voltage of less than 100 volts, which make it a very promising potential material for field emission applications.
Arc discharge is widely used in forming carbon nanotubes, but it is not suitable for mass production of carbon nanotubes at a low cost, and further more it's difficult to control a structure of carbon nanotube arrays by this method. To overcome these problems, chemical vapor deposition has been developed. Representative chemical vapor deposition methods include thermal chemical vapor deposition (CVD) (Appl. Phys. Lett. 67, 2477, 1995), microwave plasma-enhanced chemical vapor deposition (MPECVD) (App. Phys. Lett. 72, 3437, 1998), and ion beam irradiation (Appl. Phys. Lett. 69, 4174, 1996).
However, forming carbon nanotube arrays used for field emission has other difficulties. The carbon nanotubes formed by chemical vapor deposition methods usually twist at the top thereof, which reduces the efficiency of field emission. Consequently, a method for trimming the carbon nanotubes is necessary. Unfortunately the method for trimming the carbon nanotubes is usually complex or costly. Further more, chemical vapor deposition methods are unsuitable for making uniform and large-area field emission cathode.
Another method for making a carbon nanotube array for field emission includes the steps of printing a layer of grease or emulsion with a predetermined quantity of carbon nanotubes dispersed therein, and peeling off the layer of grease to expose the carbon nanotubes so that they can emit electrons. However, the step of peeling usually destroys the carbon nanotube array as well as the carbon nanotubes. Thus, another way for making a carbon nanotube array is desired.
U.S. Pat. No. 6,616,495, issued to Toshiyuki Tsuboi, discloses a method for forming a carbon nanotube film comprising the steps of preparing a suspension having a solvent and coarse carbon nanotubes dispersed therein, disposing a substrate in the suspension, the substrate having an exposed portion patterned into a predetermined shape, and depositing a carbon nanotube film on the exposed portion of the substrate by evaporating the solvent. The carbon nanotube film can be used for a field emission cathode directly.
However, in the carbon nanotube film almost all the carbon nanotubes lie on the substrate. Carbon nanotubes emit electrons only in an axial direction thereof. Thus the carbon nanotubes lied on the substrate is not beneficial to exert field emission feature of the carbon nanotubes. Therefore, an improved carbon nanotube-based field emission cathode device which can fully exert the field emission feature of the carbon nanotubes is desired.